The present invention relates to methods and apparatus for accurately and efficiently emulating a polyphase machine, such as a three-phase machine, e.g., a motor/generator, in order to load a motor drive inverter.
Polyphase motors, such as permanent magnet, synchronous machines must be driven such that the windings thereof are energized as a function of the rotor position in order to obtain a driving torque from the machine. The windings of the polyphase motor are typically driven utilizing a motor driver inverter (inverter), which receives a direct current (DC) source of voltage and produces an alternating current (AC) source of voltage for driving the polyphase motor.
Development and testing of motor drive inverters is usually accomplished using a motoring dynamometer. For four quadrant operation, the dynamometer operates as a motor/generator and the power applied to, or taken from, the dynamometer is normally dissipated as heat. There are some systems in which the power applied to, or taken from, the dynamometer is recirculated back to the MAINS.
As an alternative to testing a motor drive inverter using a motoring dynamometer, systems have been developed in which a load inverter is used to emulate a three-phase machine (thereby emulating the characteristics of a motoring dynamometer). A motor emulator is desirable as an alternative to the dynamometer testing approach because one may test the motor drive inverter without moving parts, and if the motor emulator is properly designed, heat loss may be significantly minimized. A motor emulator inverter should accurately mimic the dynamic characteristics of a three-phase machine, at least to the extent that adequate testing of the motor drive inverter is achieved. Thus, the motor emulator should mimic the three phase back-electro-motive-force (BEMF) characteristics of a physical motor, and should operate in multiple quadrants, preferably all four quadrants of the torque versus speed curve.
Reference is made to FIG. 1, which illustrates a conventional system 10 for testing a motor drive inverter 12 (the device under test, DUT) using a motor emulator inverter 14. The system 10 operates from a three-phase source of AC power, the MAINS.
On the input side of the DUT inverter 12, the system 10 includes an AC/DC converter 16 and a DC power supply 18. The combination of the AC/DC converter 16 and the DC power supply 18 provides a bi-directional source/sink for power to/from the DUT inverter 12. For example, the potential between the +/−V1 nodes of the DC power supply 18 may be on the order of about 300 VDC. Although one skilled in the art will appreciate that the DC power supply 18 would appear to be redundant (indeed, the AC/DC converter produces DC power), the DC power supply 18 is sometimes identified as a functionally distinct circuit because it may provide battery emulation (exhibiting the time-variant charge characteristics of a physical battery).
On the output side of the DUT inverter 12, the system 10 includes the motor emulator 14 (coupled to the DUT inverter via three-phase connection) and an AC/DC converter 20. The AC/DC converter 20 provides a bi-directional source/sink for power to/from the motor emulator 14. For example, the potential between the +/−V2 nodes of the DC power supply 20 may be on the order of about 400 VDC. Note that in this example, the +/−V2 magnitude is higher than the +/−V1 magnitude. This is so to permit the motor emulator inverter 14 to operate as a motor/generator when the DUT is at or near modulation indices of 1.0. Here the motor emulation inverter must source current at voltages that are higher than the terminal voltages of the DUT inverter 12. Obviously, at lower modulation indices, where the DUT inverter 12 is producing relatively lower terminal voltages, the +/−V2 magnitude may be less than or equal to the +/−V1 magnitude.
Generally speaking, when the DUT inverter 12 is sourcing power, the AC/DC converter 16 and the DC power supply 18 source power from the MAINS to the DUT inverter 12, the motor emulator 14 operates as a motor, and the AC/DC converter 20 sinks power from the motor emulator 14 to the MAINS. Conversely, when the DUT inverter 12 is sinking power, the AC/DC converter 20 sources power from the MAINS to the motor emulator 14, the motor emulator 14 operates as a generator, and the AC/DC converter 16 and the DC power supply 18 sink power from the DUT inverter 12 to the MAINS.
Thus, the system 10 permits the recirculation of current during testing, which is an advantageous way of testing the DUT inverter 12 because relatively low amounts of power are lost through heat dissipation (as compared to motoring dynamometer systems). Among the significant disadvantages of the system 10, however, is that fact that all of the converters (i.e., the AC/DC converter 16, the DC power supply 18, and the AC/DC converter 20) must be rated at full power (with reference to rated power of the DUT inverter and/or the motor emulator 14). This is so because the system 10 recirculates current through the MAINS. Thus, for example, if the DUT inverter 12 is to be tested at 100 KW, then all of the converters in the system must be rated for at least 100 KW operation. Such full power rating requires significant component and circuit costs and also contributes to higher heat losses in the system.
Reference is now made to FIG. 2, which illustrates an alternative system 50 for testing the motor drive inverter 12 using the motor emulator inverter 14. Instead of employing separate, bi-directional DC converters (as was the case in the system of FIG. 1), the system 50 couples the respective DC sides of the DUT inverter 12 and the motor emulator 14 together. This configuration permits current recirculation without going back through the MAINS. Thus, when the DUT inverter 12 is sourcing power, the motor emulator 14 operates as a motor and recirculates current back to the DUT inverter 12. Conversely, when the DUT inverter 12 is sinking power, the motor emulator 14 operates as a generator, and the DUT inverter 12 sources current to the motor emulator 14.
The advantage of recirculating current locally, over the DC bus (as opposed to through the MAINS), is that the separate, full rated converters on each side of the DUT-emulator are not required. This reduces costs and improves efficiency. Indeed, the system 50 employs a uni-directional DC power supply 52, which only sources power in order to make up for any losses in the DUT inverter 12 and the motor emulator 14. Thus, the uni-directional DC power supply 52 need not be rated for full power (e.g., 100 KW), and could probably be rated for about 10% of full power.
The motor emulator 14 of the system 50 has been found to exhibit improved operation when it has the ability of sense when the switching transistors of the DUT inverter 12 are going to transition from state to state, without relying solely on sensing the output current of the DUT inverter 12. Sensing the commutation of the switching transistors of the DUT inverter 12 has been achieved using: (i) a set of common mode inductors coupled in series with a set of differential mode inductors; and (ii) a set of integrating capacitors, arranged in a Y configuration from respective junctions of the series coupled windings of the common mode and differential mode inductors. A Z-axis voltage sense, which is an emulation of the common mode voltage of the DUT inverter 12, is obtained at the common node of the Y configuration of capacitors. Such common mode voltage is thus available to the motor emulator 14 control circuitry, which provides information as to switch commutation within the DUT inverter 12.
The system 50 is not without disadvantages. The system 50 exhibits a relatively significant problem at modulation indices at or near 1.0. Indeed, since the same DC bus supplies both the DUT inverter 12 and the motor emulator 14, there is a limit to the modulation index at which the DUT inverter 12 may be tested. Indeed, if the DUT inverter 12 operates beyond such modulation index threshold, then the motor emulator 14 will not be able to source current to the DUT inverter 12 at voltage magnitudes that exceed the terminal voltages of the DUT inverter 12.
Depending on the PWM technique employed by the DUT inverter 12, the system 50 may exhibit a related problem at all modulation indices. This related problem will surface when, for example, the DUT inverter 12 employs a so-called discontinuous PWM switching technique. Consider, by way of example, a discontinuous PWM inverter employing a full, three-phase bridge of six switching transistors. In such an inverter, one of the six switching transistors will be continuously ON for a given 60 degrees of the output frequency of the inverter, with the remaining switching transistors being modulated via PWM to achieve the desired AC voltage and current profiles on all phase lines. In a next 60 degree interval, a different one of the switching transistors will be ON, and so forth. Since one transistor is continuously ON during a given 60 degree interval (irrespective of the modulation index), the motor emulator 14 must be capable of producing a higher magnitude voltage on the same phase line associated with the ON transistor, otherwise, there would be no way for the motor emulator 14 to control the current on that line.
Accordingly, there are needs in the art for new methods and apparatus for loading a motor drive inverter with an inverter for emulating a polyphase machine, e.g., a motor/generator.